2 Testing for the presence of alcohol

As you have learned throughout this course, alcohol is a drug that has a depressive effect on the nervous system and excessive exposure can lead to various short-term and long-term effects.

Alcohol is not digested when absorbed in the gut, nor is it chemically changed in the bloodstream. As the blood goes through the lungs, some of the alcohol moves across the membranes of the lungs’ air sacs (alveoli) into the air in the lungs because alcohol is volatile. The concentration of the alcohol in the alveolar air is related to the concentration of the alcohol in the blood. As the alcohol in the alveolar air is exhaled, it can be detected in the breath. This allows for a non-invasive analysis of blood alcohol concentration (BAC) at the roadside using an instrument known as a breathalyser, an example of which is illustrated in Figure 6.

Figure _unit9.2.1 Figure 6 An example of a breathalyser used for roadside BAC testing

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This is a photograph of a breathalyser.

Figure 6 An example of a breathalyser used for roadside BAC testing

If a positive result to a roadside breathalyser test is obtained, a second sample will be taken for legal purposes at a police station to confirm the result. This further test is usually performed on either a blood or a urine sample. As the excretion rate of alcohol by the body can be calculated, this second measurement can be extrapolated backwards (retrograde) to the time of the first measurement, thereby confirming the result of the initial roadside test.

In this final part of the course, you are going to look at the chemistry of the breathalyser – the same methodology as that used in versions of the kit you can buy yourself over the counter.